Alpha-lipoic acid is 1,2-dithiolane-3-pentanoic acid (thioctic acid).
As a coenzyme of alpha-ketonic acid dehydrogenases, alpha-lipoic acid is widespread in plants and animals; the naturally occurring form has the R-configuration. However, when "alpha-lipoic acid" is referred to below, it is always an alpha-lipoic acid of unknown stereochemical composition.
Alpha-lipoic acid is pharmacologically active and has anti-inflammatory and antinociceptive (analgesic) as well as cytoprotective properties.
A series of salts of alpha-lipoic acid are known, for example salts of alpha-lipoic acid with optically active bases such as salts with the basic amino acids arginine and lysine. (See Spanish patent No. 313,056). Neither the reaction of L-arginine with D,L-alpha-lipoic acid nor the reaction of DL-lysine with D,L-alpha-lipoic acid yields separable diastereomeric salt pairs.
Known syntheses of enantiomerically pure alpha-lipoic acid always include chiral primary steps which are split in the course of the synthesis. In Walton, Wagner, Peterson, Holly and Folkers (J. Amer. Chem. Soc. 76 (1954), page 4748 et seq.) the intermediate product 7-carbethoxy-3-acetylthioheptanoic acid is split with L-ephedrine. Another process published by the same authors in J. Amer. Chem. Soc. 77 (1955) page 5144 involves the same intermediate product. Japanese patent 7970 describes the addition compound of alpha-lipoic acid with beta-cyclodextrin without mentioning the separation into enantiomeric compounds.